1. Field of the Invention
This invention relates to an inductor, particularly to a dry inductor having no iron core, which inductor comprises two or more concentric cylindrical coils extending about each other and defining intervening cooling air gaps. The coils are electrically connected in parallel and have numbers of turns which decrease in a radially outward direction.
2. Description of the Prior Art
In such inductors, the total conductor cross-section required is divided into a plurality of individual conductors which are insulated from each other. That design minimizes the eddy current losses of the inductor. Such inductors are used mainly in power engineering as compensating inductors, filter inductors and series-connected inductors.
The basic design of concentric coils which extend about each and are electrically connected in parallel is known from BBC-Nachrichten of July/August 1930 and has been described in German patent publication No. 1,294,541. From these publications it is apparent that the distribution of the current to the coils connected in parallel depends on the numbers of turns of the individual coils and the numbers of turns generally decrease from the innermost coil to the outer one. If all coils had the same number of turns, they would have different inductivities so that the current would not be adequately distributed in practice.
If identical conductors are used in the several coils, the different numbers of turns will result in different axial dimensions of the coils. The axial dimension of a coil will subsequently be referred to as the height of the coil. Owing to the different heights of the coils, the axial voltage gradients at the parallel-connected coils vary and impose electric stresses on the structural elements disposed between adjacent coils.
That disadvantage could be eliminated by the use of conductors having different cross-sectional areas in different coils so that all coils have the same height. But that concept is not economical because the several conductors having different cross-sectional areas are needed in relatively small quantities, regardless of whether stranded conductors, round wires or twisted conductors are used.
The distribution of the total conductor cross-section which is required to a plurality of coils connected in parallel is not sufficient to keep the eddy current losses within economical limits. For this reason, the conductor cross-section of each coil must be divided into a multiplicity of individual wires which are insulated from each other.
In a known design, the concept of using concentric coils which are connected in parallel is extended in that each of these coils is divided into concentric windings having graded numbers of turns. Each of these windings consists of insulated wire and the windings are directly wound one on the other. Owing to the extremely high mutual inductivities, the distribution of current to such parallel-connected windings directly wound one on the other requires that the windings have such numbers of turns that the end turns extend only around part of the periphery of the winding and it is usually necessary to connect the ends of the windings by a current-distributing spider of conductive material to a common terminal. That requirement cannot be met with conventional current-distributing spiders having 6, 8 or even 12 arms and it is difficult to make spiders having a larger number of spider arms. For this reason, an irregular distribution of current to the several windings wound one on the other must be tolerated. As a result, the current density is not homogeneous within each multi-winding coil so that the conductors are not economically utilized and a uniform temperature in each coil cannot be obtained.
It is also known to divide the conductor cross-section of each coil by the use of a twisted conductor consisting of a plurality of insulated rectangular individual conductors. Each of the individual wires of the twisted conductor aasumes different positions relative to the axis of the twisted conductor along the latter so that each individual wire lies in different induction zones of the coil. Nevertheless, all individual conductors are subjected to the same average induction conditions so that the current is uniformly distributed to all individual conductors of the twisted conductor. Besides, the twisted conductor may be composed of individual conductors having different dimensions so that cylindrical coils having different numbers of turns may have the same height. But that concept is also uneconomical because a large number of conductors having different cross-sectional areas are required in relatively small quantities. Besides, the magnetic field in the boundary zones of the coil has a strong radial component, which is transverse to the rectangular individual conductors of the twisted conductor and gives rise to heavy eddy current losses therein so that an economically optimum utilization of the conductors is not possible.